nixosTests.systemd-confinement: fix script format

https://hydra.nixos.org/build/142591177/nixlog/30

ZHF: #122042

nixos/tests/systemd-confinement.nix

···
                   "chroot-exec chown 65534 /bin",
               )
               machine.succeed(
-                  'test "$(chroot-exec id -u)" = 0', "chroot-exec chown 0 /bin",
+                  'test "$(chroot-exec id -u)" = 0',
+                  "chroot-exec chown 0 /bin",
               )
         '';
       }